Objective criteria do not exist as yet for determining the quality, nature or, quite generally, the condition of a varnished or enameled workpiece surface, in other words a surface to which a lacquer or varnish coat has been applied. Such surfaces are simply examined visually by trained staff and then classified subjectively according to characteristics such as "orange peel skin", "cloudiness" or "unsteadiness". The characterization cannot be but inaccurate, as these "florid" terms themselves suggest.
The reason why this kind of surface examination is not reliable is due to the fact that it depends a lot on the personal impression and experience of the examining person. Such results can never be unambiguous because objective criteria are missing.
A device for measuring the "orange peel skin effect" or disturbance of the flow on surfaces is known from catalog 94/08 by Messrs. BYK-Gardner GmbH, of Geretsried, Germany and also from DE-A-41 27 215. The device simulates the visual observation of the surface. As with the eye, the wavy brightness pattern on the surface is scanned optically. A measuring instrument directs a light beam against the surface and measures the reflection. The resulting brightness profile is divided into long wave and short wave components, this long wave and short wave characterization serving as an indirect measure of analyzing the surface. The catalog is silent on the further processing of the measurement signals. It is a disadvantage of the method described that the measurement is performed by way of the reflection of light so that it is applicable only with reflecting surfaces. Surfaces having mat finishes cannot be characterized.
DE-A-41 30 068 describes an optical reflection measuring device which detects the intensity of light reflected from a surface and calculates the local frequency spectrum of the reflected intensities. A comparison of the local frequency spectrum, or meaningful parts thereof, with a theoretical or desired value is made in order to verify the surface roughness of a workpiece. The method described in this prior publication is not suitable for determining the physical appearance of a surface in respect of the effects specified above.